MR Device Synthetic Free Layer Structure
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Solution Overview
Problem
Magnetic tunnel junctions (MTJs) face challenges in achieving a high signal-to-noise ratio due to high magnetostriction coefficients in CoFeB-based free layers, which are exacerbated by magnetic noise from orange-peel coupling and magnetostatic coupling, limiting the improvement in signal amplitude.
Innovation Solution
A multilayer laminate free layer structure with at least three ferromagnetic layers separated by antiparallel coupling layers, where the central layer has a magnetization direction antiparallel to the outer layers, including CoFeB for low Hc and negative lambda, and optionally NiFe for strong coupling, with Ru or other materials for coupling, to reduce magnetostriction and enhance MR ratio.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of operation
If CoFeB-based free layer is used to achieve low Hc and negative lambda, then magnetic softness is improved, but magnetostriction coefficient increases causing high magnetic noise
Solution Approach 1:
The free layer is divided into multiple sub-layers (CoFeB layer, NiFe layer, and coupling layer) to separate the functions of achieving magnetic softness and reducing magnetostriction. The CoFeB layer provides low Hc while the NiFe layer with negative lambda reduces overall magnetostriction, and the coupling layer manages the interaction between them.
Solution Approach 2:
The invention uses a composite free layer structure combining CoFeB and NiFe materials with different magnetic properties. CoFeB contributes low coercivity while NiFe contributes negative magnetostriction, creating a composite structure that achieves both magnetic softness and reduced magnetostriction coefficient.
2Reliability
If orange-peel coupling and magnetostatic coupling are used to couple magnetic layers, then magnetic coupling is achieved, but magnetic noise increases reducing signal-to-noise ratio
Solution Approach 1:
A coupling layer (Ru, Rh, Cu, Ag, or Cr) is introduced as an intermediary between the CoFeB layer and NiFe layer. This coupling layer mediates the magnetic interaction, providing controlled exchange coupling while reducing direct orange-peel coupling and magnetostatic coupling that generate magnetic noise.
3Device complexity
If single layer free layer structure is used, then device complexity is low, but signal-to-noise ratio is limited due to high magnetostriction
Solution Approach 1:
The free layer is segmented into multiple functional sub-layers, each with specific thickness and composition, to achieve better performance. The CoFeB layer (5-20 nm) provides magnetic softness, the NiFe layer (2-10 nm) reduces magnetostriction, and the coupling layer (1-5 nm) manages magnetic interaction between them.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The solution achieves a high signal-to-noise ratio with reduced magnetostriction, maintaining magnetic softness and enhancing the MR ratio, as demonstrated by specific TMR stack samples with adjusted layer thicknesses and compositions.
Implementation Method 1
at least three ferromagnetic layers separated from one another by antiparallel coupling layers
Implementation Method 2
including CoFeB for low Hc and negative lambda
Implementation Method 3
enhancing the MR ratio
Data Source
AI summary
A magneto-resistive device having a large output signal as well as a high signal-to-noise ratio is described along with a process for forming it. This improved performance was accomplished by expanding the free layer into a multilayer laminate comprising at least three ferromagnetic layers separated from one another by antiparallel coupling layers. The ferromagnetic layer closest to the transition layer must include CoFeB while the furthermost layer is required to have low Hc as well as a low and negative lambda value. One possibility for the central ferromagnetic layer is NiFe but this is not mandatory.